1. Field of the Invention
The invention relates to an exhaust pipe structure in which an exhaust passageway of exhaust gas discharged from a vehicle-installed engine is provided with at least one muffler (silencer).
2. Description of the Related Art
Vehicles are equipped with an exhaust apparatus for discharging exhaust gas produced in association with the operation of an engine. Such an exhaust apparatus generally adopts an exhaust pipe structure in which a catalytic converter, a subsidiary muffler, and a main muffler are disposed on an exhaust passage in that order from the engine side toward a downstream side, and adjacent components are interconnected by a small-diameter exhaust pipe (e.g., see the 4th to 5th columns and FIG. 1 in Japanese Utility Model Publication No. 5-4501). As for the components forming the exhaust pipe structure, the catalytic converter is used for purification of exhaust gas, and the subsidiary muffler and the main muffler are used to reduce the exhaust gas temperature and pressure and to reduce the exhaust noise. The subsidiary muffler is employed for the purposes of, for example, solving the problem of insufficient silencing capability of the main muffler in the case of a long exhaust passageway or the like. The main muffler normally adopted has an outer cylinder that forms an outer shell portion, and that has a large sectional area and a short length.
However, if the above-described exhaust pipe structure is applied to a low-floor vehicle, the location of installation of the main muffler is limited to a rearward portion of the vehicle. That is, the main muffler having the above-described configuration inevitably has a large sectional area and therefore a large size so as to achieve a sufficiently high silencing capability. It is difficult to install the large-size main muffler at a site other than a rearward portion of the vehicle. Thus, although the large-size main muffler can be installed a rearward portion of the low-floor vehicle in a tolerable fashion, the height of the floor of the main muffler-installed location (the rearward portion of the vehicle) cannot be reduced substantially to the same level of the floor of the other portions. Since the installation of the main muffler requires a relatively high floor level as mentioned above, it becomes difficult, for example, in the case of a low-floor sedan, to provide in a rearward portion of the vehicle a large luggage space that has a sufficiently great dimension in the vertical direction. Furthermore, in the case of a low-floor minivan equipped with a third row seat, it is difficult to provide a large third row seat-mounting space having a sufficiently great dimension in the vertical direction in a rearward portion of the vehicle. Therefore, it is difficult to provide a third row seat with spacious comfort.
In the above-described exhaust pipe structure, vibration in a vibration mode in which nodes exist in heavy components, such as the catalytic converter, the subsidiary muffler, the main muffler, etc., and loops exist in the exhaust pipe (exhaust pipe elastic resonance) occurs at about 150 to 200 Hz, due to vibration produced in association with the operation of the engine. The vibration is likely to cause concentration of stress in junctions between the heavy components and the exhaust pipe since, in the junctions, the sectional shape sharply changes. In order to prevent the stress concentration from causing deformation or the like, it is necessary to increase the strength of the junctions. Measures for increasing the strength of junctions are needed particularly if the exhaust pipe is welded to a heavy component.